They included Michael Spencer, a former Tory treasurer, but not Peter Cruddas, who quit from the same role after plunging the Conservatives into a damaging cash-for-access row.

Those invited were old friends of the Prime Minister who just happened to have donated money to the party, the source added.

Labour leader Ed Miliband has demanded full disclosure of which Tory donors had visited Downing Street or Chequers, Mr Cameron's country residence, since May 2010 and what policy representations they had made.

The Tory treasurer and fundraiser, himself a donor, urged reporters posing as wealth fund executives to give more than £250,000 in return for meetings with senior ministers.

He claimed those making such donors, classed as "premier league" could raise issues with ministers and feed their concerns into a Downing Street "policy committee".

Mr Cruddas resigned after his claims were exposed by The Sunday Times and denied that party donors could in fact improperly influence ministers. The matter has been reported to the police.

Peter Cruddas resigned after The Sunday Times exposed his claims Credit: The Sunday Times

Mr Cameron insisted that was "not the way" the Conservative Party raised money and promised an internal inquiry to ensure it would not happen again.

What happened is completely unacceptable. This is not the way that we raise money in the Conservative Party, it shouldn't have happened.

It's quite right that Peter Cruddas has resigned. I will make sure there is a proper party inquiry to make sure this can't happen again."